Internal energy is a term used in thermodynamics denoting the total potential as well as kinetic energy of a body with well-defined boundaries. This kinetic energy is due to the motion of molecules which can be rotational, translational and vibrational as well. The potential energy accounting for the internal energy of the thermodynamic system is the energy due to the vibrtational and electric energy possessed by atoms within crystals of molecules. The internal energy of a blackbody or electromagnetic radiation can also be calculated. Internal energy can also be defined as the functioning state of a system. Internal energy is also an extensive quantity. Even though, the SI unit of energy is joule, other conventional units are also use for internal energy such as calorie for heat. Internal energy is the total sum of each and every microscopic forms of energy within a thermodynamic system.

Internal energy is linked with the molecular structure and also with the level of molecular activity including both the potential as well as the kinetic energy. The internal energy of a thermodynamic system does not encompass the rotational or translational energy of a body in general. Internal energy also does not include the mass-energy equivalent. Internal energy also excludes and does not account for the potential energy that a body may have due to its position in an external electrostatics or gravitational field. However, the potential energy due to induced magnetic or electric dipole does account for internal energy.